1. Field of the Invention
This invention relates to dimming of gas discharge lamps.
2. Description of the Related Art
Continuous dimming of lighting is desirable for a number of reasons. It can change the "atmosphere" in an illuminated space; it can provide different lighting levels for different activities in the space; and it can adjust the electrical lighting in a space to compensate for variations in natural lighting.
Various means are known for controlling most light sources. One such lighting control operates by periodically blocking the supply voltage to the light source in accordance with a phase delay during each half cycle of an AC applied voltage. These "phase control dimmers," as they are commonly referred to, generally employ a thyristor, such as a silicon controlled rectifier (SCR) or, more commonly, a triac as an electronic blocking device or switch. Gate turn off devices, and bipolar and MOSFET transistors have also found limited use in phase control circuits, but triacs are prevalent.
A triac normally has three terminals--a cathode, an anode and a gate (or control terminal). Current may be injected into or drawn out of the gate to fire the triac (i.e., make it bi-directionally conductive). Once fired, a triac will remain conductive until the current through it drops below a certain level known as the holding current. By firing the triac at some adjustable phase delay after each zero crossing of the (generally sinusoidal) applied voltage, one can vary the brightness of a light source.
One type of commonly used light source that has been historically difficult to dim is the gas discharge lamp. A gas discharge lamp is generally an elongated gas-filled (usually low pressure mercury vapor) tube that has electrodes at each end. The steady-state operation of a gas discharge lamp is as follows: Hot electrodes thermionically emit electrons into the tube. A voltage applied between the electrodes accelerates the electrons toward the anode. In route to the anode, the electrons collide with gas atoms to produce positive ions and additional electrons. The electrons continue to stream toward the anode and the positive ions toward the cathode, sustaining an electric arc in the tube and further heating the electrodes. (If the applied power is AC, the electrodes reverse polarity during each half cycle.) Because each collision produces additional electrons, increases in the arc current cause the impedance of the lamp to decrease, a characteristic known as "negative resistance." Operation of the lamp is inherently unstable, due to this negative resistance characteristic, and current between the electrodes must be limited to avoid damaging the lamp.
Typically, fixtures designed to operate gas discharge lamps include a current limiting element (a "ballast") in series with the lamp. A ballast is an impedance and can simply be a resistor. More commonly, a reactive impedance, such as an inductor or a combination of inductors and capacitors, is used, since it is a more efficient ballast when the applied potential is AC. A ballast limits the current through the lamp during steady-state operation but provides a high voltage during lamp start-up to "strike" the arc.
During lamp start-up, the electrodes of a conventional ("cold cathode") gas discharge lamp are cold and there are almost no free electrons in the tube. The impedance of the lamp is very high, and the voltage required to initiate or strike an arc far exceeds that required to sustain the arc. The exceedingly high voltage that is required to strike an arc between the cold electrodes can damage them and reduce the life of the lamp. To reduce the damaging effects of lamp start-up and increase thermionic emission of electrons during steady-state operation, many lamps, such as the "rapid start" lamp which is widely used in the U.S., have electrodes that are heated by a separate circuit that operates independently from the arc current. The electrodes of such lamps typically consist of a tightly wound coil of tungsten coated with a thermo-emissive material. Current flows through the coil between terminals on either side of the coil to raise its temperature and cause it to thermionically emit electrons.
Ballasts designed to operate rapid start lamps typically consist of a leakage reactance auto-transformer with separate windings to provide low voltage across each electrode coil--to heat the coil--and high voltage between the electrodes--to induce an electric discharge through the lamp. Leakage inductance in the high voltage winding limits the current through the lamp during steady-state operation. The low voltage windings provide additional power to heat the electrodes during lamp operation to ensure sufficient thermionic emission. In this specification and appended claims, we refer to a leakage reactance auto-transformer as a leakage auto-transformer.
If a rapid start lamp is dimmed using a phase controlled voltage and rapid start ballast, not enough voltage is provided across the electrode coils at low power levels resulting in insufficient thermionic emission at the cathode. This limits low end dimming capability to approximately 60 percent of full light output. Furthermore, the temporary absence of arc current during the phase delay necessitates restriking the arc during each half cycle, which shortens lamp life. For these reasons, phase control dimmers generally have not been used for full range dimming of rapid start lamps.
A number of control systems have been devised for the control of gas discharge lamps. Luciano Di Fraia suggested, in a paper presented to the IES 1980 annual conference, that gas discharge lamps may be dimmed by varying the frequency of the voltage provided to the lamp and a series-connected rapid start ballast. As the frequency is increased, the power transferred through the high voltage winding of the ballast decreases, because of its high leakage inductance. The low voltage windings, however, have significantly less leakage inductance and provide a voltage across the electrode coils that keeps the electrodes heated. Unfortunately, variable frequency controls are expensive and inefficient. They also produce a high level of electromagnetic interference if switching transistors are used.
U.S. Pat. Nos. 3,619,716 and 3,731,142, incorporated herein by reference, teach dimming of gas discharge lamps with a single high-frequency power switching device and a pulse forming network connected across the lamp. By keeping the conduction time of the switching device short compared with the lamp arc time-constant, lamp current runaway is avoided. A pulse forming network stores energy and allows it to circulate through the arc when the switching device is not conductive, thus keeping the arc struck. These inventions have been embodied in the Hi-lume.RTM. electronic dimming ballast, sold by Lutron Electronics Co., Inc. It operates as follows: Control circuitry rectifies and filters voltage dropped across a current sensing resistor placed in series with the lamp arc current, and compares this voltage with the dimming control voltage input. The duty cycle of the single power switching device (a switching transistor) is adjusted until the voltage across the sensing resistor equals the dimming control voltage. The use of an accurate servo feedback loop, which directly monitors the lamp arc current, results in very stable dimming capability over a range in excess of 100 to 1 light output ratio.
Oy Helvar, of Helsinki, Finland, manufactures an electronic dimming ballast that rectifies a phase controlled voltage and converts it into a high-frequency voltage which is applied between the lamp electrodes. The light output of the lamp is determined by the power supplied to the electronic ballast by a phase control dimmer. Although the electronic ballast may offer certain advantages, it suffers from a number of drawbacks, such as high voltage (750 V) placed across the phase control dimmer and insufficient striking voltages at low power levels.
There is a need for a gas discharge lamp dimmer that works with standard magnetic ballasts and that provides dimming control over a wide illumination range.